1. Field of the Invention
The present invention generally relates to an optical scanning apparatus. More specifically, the present invention is directed to such an optical scanning apparatus capable of correcting positional shifts contained in plural images to be synthesized, which is used in an image forming apparatus in which a plurality of optical beams are separately scanned over a photosensitive member, and a plurality of images formed on this photosensitive member are synthesized with each other to output a single synthesized image.
2. Description of the Related Art
Conventionally, images forming apparatuses are used in printers and copying machines. In these image forming apparatuses, optical beams which are modulated in response to images to be formed are scanned over photosensitive members so as to form electrostatic latent images on these photosensitive members, so that desirable images are formed on the photosensitive members. Very recently, since these electronic appliances are manufactured in digital modes and also color modes, the image forming apparatuses with employment of the above-explained structures are widely employed in these digital/color electronic appliances. A color image may be formed in such a manner that, for example, images having different four colors (e.g., C, M, Y, K) are sequentially formed on a photosensitive member, while these four color images are overlapped with each other on a single photosensitive member. However, this color image forming operation would own such a problem that a lengthy time is required until the desirable image is finally formed.
To avoid this problem, a so-called xe2x80x9ctandemxe2x80x9d type image forming apparatus has been proposed. In this tandem type image forming apparatus, a plurality of photosensitive members are provided, the respective photosensitive members are scanned/exposed at the same time by a plurality of optical beams to form images having different colors from each other on the respective photosensitive members, and then these images having the different colors are overlapped with each other on the same transfer member. As a result, a desirable color image can be formed. Since this tandem type image forming apparatus simultaneously forms the images having the respective colors at the same time, the time required to form the color image can be largely shortened.
However, in this tandem type image forming apparatus, when the images having the different colors are overlapped with each other, these are certain possibilities that the positional shifts are easily produced due to the fluctuations in the optical characteristic of the optical beams corresponding to the respective color images. Also, these positional shifts of these images may be apparently and visually recognized in the color image. Accordingly, the color shifts must be corrected in order to obtain a high grade image. While the image forming apparatus is manufactured, even when the respective components of this image forming apparatus are adjusted so as to cancel the color shifts, if the peripheral environments such as temperatures are varied, then the color shifts are produced due to some reason, for instance, the arranging positions of the optical components.
Japanese Patent No. 2748971 describes the following color shift corrections employed in the tandem type image forming apparatus. That is, the sensor for sensing the positional shifts of the beams are provided in the vicinity of the respective photosensitive drums. The sensor sense the beam positions along the sub-scanning direction so as to control the write timing along the sub-scanning direction, so that the color shifts along the sub-scanning direction can be corrected.
As another color shift correcting method, the following correcting method is known. That is, while the patterns (for example, mark xe2x80x9c+xe2x80x9d) capable of easily being sensed the color shifts are formed on the photosensitive members, these patterns are read out by the image reading apparatus to be sensed the color shifts. Then, the color shifts are corrected by changing the positions of the beams entered into the photosensitive drums by moving the reflection mirrors, or by controlling the write timing along the sub-scanning direction.
However, the conventional color shift correcting method described in Japanese Patent No. 2748971 owns the following drawback. That is, since the sensor for sensing the positional shift amounts along the sub-scanning direction are provided in correspondence with the photosensitive drums, a positional relationship between a newly replaced photosensitive drum and the relevant positional shift-amount sensor must be adjusted when the old photosensitive drum is replaced by this new photosensitive drum. As a result, the replacing work of the photosensitive drum unit becomes cumbersome. Also, since the positional shift-amount sensor are arranged in the vicinity of the photosensitive drums, there is another problem that these positional shift-amount sensors are contaminated by dust and the like.
Also, in the case that the color shifts are sensed based upon the color shift sensing patterns, since the color shift sensing patterns must be formed on the photosensitive drums, the complex control operation is required. Also, the image reading apparatus for reading the color shift sensing patterns is expensive. The image forming apparatus is made in high cost, and furthermore, the overall dimension of this image forming apparatus is increased so as to secure the space for arranging this image reading apparatus.
The present invention has been made to solve the above-explained problems. This invention provides such an optical scanning apparatus capable of correcting a positional shift contained in a plurality of images with employment of a simple and low-cost arrangement, while these plural images are synthesized with each other to thereby output a synthesized image as a single image.
To achieve the above-explained advantages, an optical scanning apparatus, according to the present invention, is featured by that an optical scanning apparatus used in an image forming apparatus for scanning a plurality of optical beams on photosensitive members and for synthesizing a plurality of images formed on the photosensitive members to output a synthesized image as a single image, the optical scanning apparatus comprising: a light source for projecting the plurality of optical beams; deflector for deflecting the optical beams; first beam sensor for sensing positions of the respective optical beams along an optical beam scanning direction; second beam sensor for sensing positions of the respective optical beams along a direction intersected with the scanning direction; and modulation controller for controlling a modulation of each of the plural optical beams projected from the light source based upon the sense results obtained from the first beam sensor and the second beam sensor in such a manner that a positional shift occurred when a plurality of images formed by the respective optical beams are synthesized with each other is suppressed.
In an image forming apparatus in which the optical scanning apparatus according to the present invention, while a plurality of optical beams may be scanned over the photosensitive members to form a plurality of images, these plural images are synthesized with each other to output the synthesized image as a single image. As a consequence, for example, when these plural images are such image having different colors from each other, these plural images may be synthesized with each other and then the synthesized image becomes a multi-color image (when colors of plural images are K, Y, M, C, a full color image is outputted). It should be noted that a total number of photosensitive members might be basically selected from 1, or plural number. However, when an image forming apparatus is arranged in such a way that a plurality of photosensitive members are provided and images are formed on the respective photosensitive members at the same time by way of a plurality of optical beams (namely, tandem system), the time required to finally output the synthesized image may be preferably shortened. The optical scanning apparatus may comprising a light source for projecting the plurality of optical beams, and the deflector for deflecting the respective optical beams. It should also be noted that a total number of deflector may be basically selected from 1, or plural number. When an optical scanning apparatus is arranged by that a plurality of optical beams are deflected by employing a single deflector, this optical scanning apparatus may be made compact and furthermore, owns such a merit that the complex mechanism (for example, a rotation phase of a motor is controlled).
Also, there may be provided the first beam sensor for sensing the positions of the respective optical beams along an optical beam scanning direction, and the second beam sensor for sensing the positions of the respective optical beams along the direction intersected with the scanning direction. The modulation controller controls the modulation of each of the plural optical beams projected from the light source based upon the sense results obtained from the first beam sensor and the second beam sensor in such a manner that the positional shift occurred when a plurality of images formed by the respective optical beams are synthesized with each other is suppressed.
The positions of the optical beams sensed by the first beam sensor and the second beam sensor are made in correlation with the optical beam irradiation positions on the photosensitive members. As a consequence, the positional shifts of the plural images formed by the respective optical beams along both the optical beam scanning direction and also the direction intersected with this scanning direction may be corrected. If a color image is formed, then a color shift (deviation) may be corrected. Also, in the case that a positional shift is produced in a plurality of images formed by the respective optical beams due to a change in a peripheral environment, there is another change in the positional relationships among these optical beams sensed by either the first beam sensor or the second beam sensor. As a consequence, the positional shift of the images caused by the change in the peripheral environment can also be corrected.
Also, since the positional shifts are indirectly detected by sensing the positions of the respective optical beams in the optical scanning apparatus of the first aspect, the expensive image reading apparatus for reading the color shift sensing patterns are not required to be positioned in the vicinity of the arranging positions of the photosensitive members. As a result, the space defined in the image forming apparatus can be effectively utilized, and furthermore, the cost thereof can be reduced. Also, in the optical scanning apparatus of the present invention, the positional shift-amount sensor need not be employed instead of the image reading apparatus in the vicinity of the arranging positions of the photosensitive members. The replacement of these photosensitive members can be easily carried out.
As a consequence, while a plurality of images are synthesized with each other to output the synthesized image as a single image, the positional shifts contained in the plural images can be corrected with employment of the simple and low-cost arrangement. The optical scanning apparatus may be featured by further comprising: first storage for storing thereinto a first set value indicative of modulation starting timing within one scanning time period of each of the optical beams, the first set value being set in such a manner that while using as a reference such timing when a specific optical beam passes through a predetermined position within an optical beam scanning range, the positional shifts of the plural images along the scanning direction are corrected; and the modulation controller controls the modulation starting timing of each of the optical beams within one scanning time period based upon the first set value stored in the first storage while using as the reference the timing when the specific optical beam passes through the predetermined position. Since the modulation controller controls the modulation starting timing of each of the optical beams within one scanning time period based upon the first set value which is set in such a manner that the positional shifts of the plural images formed by the respective optical beams along the scanning direction are corrected, the positional shifts of the plural images to be synthesized as a single image along the scanning direction can be corrected.
On the other hand, positions of optical beams may be sensed by arranging the following structure. That is, for example, sensors are arranged in an entire range within an optical beam scanning range so as to continuously sense the positions of the optical beams. To the contrary, in such a position sensing case that modulation timing is controlled and a variation in a positional relationship among these optical beams is detected, timing at which the optical beam passes through a certain position within the scanning range may be sensed by an optical sensor arranged at this certain position. Also, since the first beam sensor senses the respective positions of the plural optical beams along the optical beam scanning direction, this first beam sensor may be constituted by such that, for instance, the above-explained optical sensor is provided with respect to each of the optical beams. In this case, the modulation controller controls the modulation starting timing of each of the optical beams within one scanning time period while using as the reference the timing when the specific optical beam passes through the predetermined position. As a consequence, although the optical sensor corresponding to this specific optical beam is required to be arranged in such a way that this optical sensor senses such timing when the optical beam passes through a position equal to an edge portion on the side of the scanning starting position within the scanning range, there is no limitation in the arranging positions as to the optical sensor corresponding to other optical beams.
As a consequence, in particular, even when the optical beams whose scanning directions are different from each other are mixed in the plural optical beams, the respective optical sensor may be arranged in such a manner that, for example, the respective optical sensor corresponding to the respective optical beams are present at the close positions. In this case, the optical scanning apparatus can be made compact. The optical scanning apparatus may be featured by that the modulation controller judges as to whether or not a variation is present in a positional relationship among the respective optical beams along the scanning direction based upon the positions of the optical beams along the scanning direction, sensed by the first beam sensor, and corrects the first set value depending upon the variation in positional relationship. The modulation controller may judge as to whether or not the variation is present in the positional relationship among the respective optical beams along the scanning direction, and corrects the first set value in response to the variation contained in the positional relationship along the scanning directions of the respective optical beams. This first set value is employed so as to control the modulation staring timing of each of the optical beams within one scanning period. As a consequence, even when the positional relationship among the respective optical beams is varied due to variations in the peripheral environment, the positional shifts of the images along the scanning directions can be firmly corrected. The optical scanning apparatus may be featured by that the first aspect is further comprising: second storage for storing thereinto a second set value indicative of modulation starting timing while using one scanning of each of the optical beams as a unit, the second set value being set in such a manner that the positional shifts of the plural images along the direction intersected with the scanning direction are corrected; and the modulation controller controls the modulation starting timing while using one scanning of each of the optical beams as a unit based upon the second set value stored in the second storage, and judges as to whether or not a variation is present in a positional relationship among the respective optical beams along the direction intersected with the scanning direction based upon the positions of the respective optical beams along the direction intersected with the scanning direction, sensed by the second beam sensor, whereby the second set value is corrected in response to the variation contained in the positional relationship. In this case, the modulation controller may control the modulation starting timing while using one scanning of each of the optical beams as a unit based upon the second set value. This second set value is set in such a manner that the positional shifts of the plural images along the direction intersected with the scanning direction are corrected. As a consequence, this optical scanning apparatus can correct the positional shifts of the plural images to be synthesized with each other as a single image along the direction intersected with the scanning direction. Also, in this case, the modulation controller judges as to whether or not the variation is present in the positional relationship among the respective optical beams along the direction intersected with the scanning direction, and corrects the second set value in response to the variation contained in the positional relationship along the direction intersected with the scanning direction of the optical beams.
This second set value is employed so as to control the modulation starting timing while using one scanning of each of the optical beams as a unit. As a consequence, even when the positional relationship among the respective optical beams is varied due to variations in the peripheral environment, the positional shifts of the images along the direction intersected with the scanning direction can be firmly corrected.